Pitch

Codex parity. `/init` walks the workspace, identifies the project type (Cargo, package.json, requirements.txt, etc.), and writes a starter `AGENTS.md` with sections the model can use to ground every future session:

• Build/test commands (auto-detected from manifest)
• Workspace layout (3-deep tree + module purpose comments where readable)
• Conventions (commit-message style from `git log`, lint config, formatter)
• Where to look first when picking up an unfamiliar task

Re-running `/init` shows a diff of the proposed update without overwriting unsaved changes.

Implementation sketch

• Reuses `utils::summarize_project` and `utils::project_tree` for the layout block.
• Shells out to `git log --oneline -20` for commit-style detection.
• Detects `Cargo.toml` / `package.json` / `pyproject.toml` for build commands.
• Writes via `write_atomic` so a bad detection doesn't trash a hand-written AGENTS.md.

Acceptance

• `/init` in a fresh project creates a coherent AGENTS.md.
• `/init` in a project that already has AGENTS.md shows a diff and asks before overwriting.
• Detected build/test commands match what's in the project manifest.
• Tests cover Cargo / npm / Python project shapes.